1. Field of the Invention
The present invention relates to a method of supporting small group multicast in mobile IP. Specifically, the invention relates to a method of supporting small group multicast in mobile IP, in which an implicit multicast packet received by a home agent is shifted to an explicit multicast packet including the Internet address of a mobile node registered as a multicast receiving party to be transmitted to a care-of address, explicit multicast routing is performed based on the current care-of address of the mobile node described in the explicit multicast packet in case where the home agent receives this explicit multicast packet, and the mobile node uses the IP address of the home agent as the originating party's address when it sends the explicit multicast packet via a multicast router of a visited network. Further, the mobile node uses a co-located care-of address as the originating party's address in case where it transmits the explicit multicast packet via the home agent, and the mobile node uses basic multicast provided by mobile IP when the loss in processes of home agents or foreign agents is judged to be more severe than the network band width loss.
2. Description of the Related Art
There will be explained below a conventional mobile IP technique and small group multicast and pointed out problems with them.
1. Mobile IP
Mobile IP allows a mobile node on the Internet to be able to Internet-communicate with its Internet address without being cut-off from Internet connection even when it changes a network to which it is connected. The mobile IP was developed by working groups of Internet Engineering Task Force (IETF) established for standardization of Internet techniques, and it specifies not only protocols with respect to one-to-one communication but also multicasting protocols corresponding to one-to-many and many-to-many communications. Technical terms used in this technique and their meanings are defined below.
Mobile node: A host or router capable of changing its connection point.
Home network: A network to which the mobile node originally belonged. The mobile node operates as a fixed node when it is plugged into the home network.
Foreign network: Networks other than the home network.
Visited network: A foreign network to which the mobile node is being currently connected.
Home agent: A router that recognizes the current connection point of the mobile node on its home network to tunnel packets coming into the mobile node.
Foreign agent: A router providing routing service on a network the mobile node visits. If the mobile node uses the foreign agent, the foreign agent receives packets the home agent tunneled and returns them to the mobile node.
Care-of address: The end point of a tunnel toward the mobile node when the mobile node resides on the foreign network. It is referred to as foreign agent care-of address when the foreign agent is used as a care-of address and co-located care-of address when the address of a local network is used as a care-of address.
Communication includes two modes of transmission and reception. A mobile node's multicast reception is described first below.
Multicast reception in mobile IP can be performed through two methods. The first method is direct reception via a multicast router of a visited network and the second one is receipt from a home agent serving as a multicast router through bi-directional tunneling between a care-of address and the home agent. Though the first method has satisfactory efficiency in terms of optimization of routing path, there is a possibility of cutting off communication due to a security policy such as ingress filtering in case of using a foreign agent care-of address. Further, when a co-located care-of address is used, it does not generate a security problem but brings about severe shortage of Internet version 4 (Ipv4) address. Accordingly, the multicast reception through the bi-directional tunneling will be used for a while when mobile IP becomes commercially available.
FIG. 1 illustrates a procedure of receiving a unicast packet in mobile IP. As shown in FIG. 1, bi-directional tunneling is performed in such a manner that a home agent 100 receives a multicast packet to be transmitted to a mobile node 300 from a specific communication counterpart 400, encapsulates the received packet in order to forward the packet to a destination, i.e., the mobile node 300, and transmits the encapsulated packet to a care-of address that is the end point of the tunnel through mobile IP tunneling. A node (foreign agent 200 in FIG. 1) having the care-of address strips the encapsulation for the mobile IP tunnel, confirms that the destination address corresponds to the mobile node 300, and then forwards the packet to the mobile node 300. The mobile mode 300 accepts the packet in unicast, removes the encapsulation and then receives it as a multicast packet. Here, the care-of address may be a foreign agent care-of address or co-located care-of address. As described above, the bi-directional tunneling is referred to as nested tunneling because encapsulation occurs twice.
FIG. 2 illustrates a case where the mobile node in mobile IP transmits a multicast packet through a multicast router of a visited network, and FIG. 3 illustrates a case where the mobile node in mobile IP transmits a multicast packet through a multicast router of a home agent.
There are two methods by which the mobile node performs multicast transmission on a foreign network. The first method is to send the multicast packet to a specific communication counterpart 400 via the multicast router (i.e. foreign agent 200 to which the mobile node is being currently connected) of the visited network, as shown in FIG. 2. The second one is to deliver the multicast packet to the counterpart 400 via the home agent 100 serving as a multicast router through bi-directional tunneling between the home agent 100 and a care-of address.
With the first method, the mobile node 300 must use a co-located care-of address as its originating party's address when it transmits the multicast packet via the multicast router 200 of the visited network on the visited network because multicast routing, differently from unicast routing, generally depends on the originating address. Accordingly, this method is not recommendable because of shortage of Internet version 4 addresses, as described above.
In the second method, the mobile node 300 generates the multicast packet using its home IP address as its originating address and transmits it to the home agent 100 through bi-directional tunneling between the home agent 100 and the care-of address, and the home agent 100 then routes the packet. Even with the second method, the mobile node 300 has to use nothing but the home IP address as the originating address of the multicast packet even if the mobile node, 300 uses the co-located care-of address because of reason similar to that of the first method.
2. Small Group Multicast
Conventional Internet multicast used addresses of Class D addresses (244.00.0239.255.255.255) that is an address range specially assigned for only Internet multicast as destination addresses. So, receiving terminals which wished to be provided with specific multicast services requested multicast routers to send packets to which multicast addresses are attached as destination addresses to them, and they was able to receive the multicast packets if the multicast routers delivered them thereto. This kind of multicast is referred to as implicit multicast. The implicit multicast has been used as a multicast technique because it has advantages that scalability can be easily maintained even if the number of receiving parties or the number of multicast routers is considerably increased and routers manage only a relatively small amount of statuses. However, there are many cases where only very small number of subscribers exists in actual situations of multicast applications. Accordingly, an explicit multicast technique has been proposed.
With the explicit multicast, every multicast packet is transmitted, including unicast addresses of all receiving parties, and intermediate routers confirm the unicast addresses of all receiving parties, duplicate and transmit the multicast packet along an appropriate path. Accordingly, the routers need not to manage members of a group and the existing unicast routing technique can be used without any change so that loads on the management of multicast statuses of the routers can be lessened. The most promising one of the explicit multicast techniques is small group multicast.
3. Problems
Multicast reception carried out by a mobile node in mobile IP generates the following problems because it uses nested tunneling.
The first problem relates to multiple nested tunnelings for multiple mobile nodes. If a plurality of mobile nodes belonging to the same home network perform multicast reception through bi-directional tunneling for the same multicast group on the same visited network, the home agent carries out encapsulation having each mobile node registered for one multicast packet the home agent received as a destination address and gathers multiple unicast packets constructed in this manner to send the gathered packets to the care-of addresses of the mobile nodes using mobile IP tunneling. Here, it can be known that, although there is one original packet, packets as many as the mobile nodes are transmitted in order that the original packet may go to the foreign network from the home agent. That is, though a payload is multicast, the path between the home agent and the foreign network cannot realize unification of overlapping traffic, an advantage of multicast, and the network efficiency is decreased because of nested tunneling rather than the unicast reception.
Secondly, there is a limit to the originating address generated in multicast transmission. Since multicast routing depends on the originating party's address, the co-located care-of address must be used as the originating address when the multicast transmission is carried out via the multicast router of the visited network and only the home IP address must be used as the originating address in case of transmission through the home agent.
The third problem is incompatibility with explicit multicast. Existing mobile IP does not consider the explicit multicast technique so that it could not service the explicit multicast even when explicit multicast packets arrived at a home network. To solve this problem, a node performing explicit multicast shifts all of explicit multicast packets to unicast packets to transmit them to the home network. As a result, mobile Internetwork becomes an ineffective network incapable of employing any of advantages of explicit multicast and implicit multicast.